1. Field of the Invention
This invention relates to a wear and impact resistant material which is comprised of polycrystalline diamond and cemented metal carbide formed at ultra high pressure and temperature.
As used in the following disclosure and claims, the term "polycrystalline diamond" is intended to refer to the type of material which is made by subjecting individual diamond crystals to ultra high pressure and temperature such that intercrystalline bonding occurs. Generally, a catalyst/binder material is used to ensure adequate intercrystalline bonding. This material is also often referred to as "sintered diamond" in the art. Also in the following disclosure and claims, the term "precemented carbide" is intended to refer to the type of material resulting when grains of a carbide of one of the group IVB, VB, or VIB metals is pressed and heated (most often in the presence of a binder such as Co, Ni, or Fe and various alloys thereof) to produce solid carbide pieces possessing high toughness. The most common and readily available form of precemented carbide is tungsten carbide containing a cobalt binder.
2. Prior Art
In several applications, polycrystalline diamond has displayed particular advantages over single crystal diamond. In particular, polycrystalline diamond is more impact resistant than single crystal diamond. Due to its extremely high modulus of elasticity, as well as its specific planes of cleavage in which relatively low forces can cause fracturing of the crystal, single crystal diamond has relatively low impact resistance. Polycrystalline diamond, which is made up of randomly oriented individual crystals, alleviates problems caused by the planes of cleavage in the single crystal form. However, polycrystalline diamond is still relatively low in impact resistance because of the high modulus of elasticity of diamond. This low impact resistance is a problem because in many applications polycrystalline diamond "wears" not from atom by atom shearing, but rather from fracturing and spalling occurring at both macro and microscopic scales.
The relative brittleness of polycrystalline diamond was recognized early, and as a result the first commercially available polycrystalline diamond products included a metallic backing layer or substrate bonded directly to the diamond layer, as shown in U.S. Pat. No. 3,745,623. The most common form of this "composite compact" to date has been a planar disc of polycrystalline diamond "grown" directly onto a precemented disc of tungsten carbide during a press cycle. However, this arrangement, in which the polycrystalline diamond layer is supported by a single precemented carbide mass or similar substrate, possesses a number of limitations.
One problem has been the limitation on the design of polycrystalline diamond tools to those configurations in which the diamond layer can be adequately supported by the carbide substrate. Although some work has been done to expand the applications (see for example U.S. Pat. No. 4,215,999 where a cylinder of polycrystalline diamond is grown around a core of precemented carbide) there are conceivable uses for polycrystalline diamond in tools which are difficult or impossible to implement with a composite compact because of the need to provide a substrate of precemented carbide for support. For example, rotary tools such as miniature grinding wheels and drills which need to be symmetrical about a line and in which the working faces are subject to tangential forces have not been commercially implemented.
Another problem arises because the precemented carbide substrate has a higher coefficient of thermal expansion than that of the polycrystalline diamond. Because the bond between the diamond layer and the precemented carbide substrate is formed when both materials are at a temperature in the range of 1,300.degree.-2,000.degree. C., stresses are created when the composite compact cools and the carbide substrate shrinks more than the diamond. Because the diamond layer is less elastic than the carbide substrate, these stresses often cause cracking in the diamond layer, either during the cooling phase or during use of the composite compact. Also, the precemented carbide substrate takes up room in the pressing cell that could otherwise be used for the formation of polycrystalline diamond.
Furthermore, when a precemented carbide mass is relied on to increase the impact resistance of polycrystalline diamond, the diamond layer is preferably relatively thin so that the diamond is never too far from its support. This restriction on the thickness of the diamond layer naturally limits both the life expectancy of the composite compact in use and the potential designs for polycrystalline diamond tools.
Yet another problem which has limited the thickness of the diamond layer in composite compacts is caused by the problem of "bridging". Bridging refers to the phenomenon that occurs when a fine powder is pressed from multiple directions. It is observed that the individual particles in a powder being pressed tend to stack up and form arches or "bridges" so that the full amount of pressure often does not reach the center of the powder being pressed. The inventor has observed that when a 1 micron diamond powder is used to make a polycrystalline diamond body which is more than about 0.06 inches thick, the polycrystalline diamond toward the center of the piece is usually not as well formed as the exterior portions of the polycrystalline diamond, a condition which can result in cracking and chipping of the diamond layer.